What the species is and where it came from
Symbiosis is any close and long-term biological interaction between two organisms of different species. In 1877 Albert Bernhard Frank used it for the partnership found in lichens. Heinrich Anton de Bary then defined it more broadly as “the living together of unlike organisms”; the source gives his year as 1879 in its opening and 1878 in its history section. The definition stayed contested for about 130 years. Some scientists restrict the word to lasting mutualisms, and others include every long-term interaction while excluding brief ones such as predation.
How it works, in terms someone would actually use
Partnerships are sorted in three ways. In obligate partnerships one or both partners depend on the other to survive, and facultative partners can subsist independently. In lichens the fungal partner cannot live alone, while the algal or cyanobacterial partner usually can. An ectosymbiont lives on the host’s surface, as head lice do on humans, and an endosymbiont lives inside, as Symbiodinium does within coral. The third sort is by effect on fitness. Competition lowers it for both partners and amensalism lowers it for one. In commensalism one partner benefits and the other is largely unaffected, in parasitism one benefits at the other’s expense, and in mutualism both benefit.
What it gets right
The framework fits real, observable pairs. Rhizobia fix nitrogen inside legume root nodules and cannot do so without the plant. Siboglinid tube worms have no digestive tract and rely wholly on bacteria that oxidise hydrogen sulfide or methane. About 80% of vascular plants worldwide form symbiotic relationships with fungi, and bacterial endosymbionts supply essential nutrients to about 10% to 15% of insects.
What it does not
The word has been defined in more than one way, and it is sometimes used in a restricted, mutualistic sense, so two writers can use it for different things. Some relationships resist a clean label. Cleaning symbiosis, where a cleaner fish eats parasites off a larger client, is putatively mutual, and biologists have long debated whether it is mutual selfishness or simple exploitation. Partners can shift roles, since fungi that are mutualists of plants can turn parasitic on weak or dying ones. Endosymbionts often have much reduced genomes, and when insects pass their endosymbiotic bacteria to offspring, deleterious mutations accumulate in the bacteria’s non-essential genes. Several larger ideas, such as a bacterial and archaeal origin for the nucleus, are described only as hypotheses.
What it changed
Symbiosis was once discounted as an anecdotal evolutionary phenomenon. The source says the evidence is now overwhelming that such associations had crucial consequences in many landmark events in evolution and in generating phenotypic diversity. Mitochondria and chloroplasts are thought to descend from bacteria taken into cells, which is called symbiogenesis. Lynn Margulis argued that evolution rests on cooperation as well as competition. In her words and her son Dorion Sagan’s, life took over the globe “by networking,” not by combat.
Who it is for, and who it is not
It suits anyone who wants to understand how species depend on each other, and students sorting the vocabulary of ecology. It also suits readers who want something to look for. The source names lichens on bare rock, barnacles on baleen whales, clownfish among anemone tentacles and hoverflies that mimic wasps. For any one pair it gives the behaviour and the relationship, not ranges, population figures or conservation status.
Is it worth your time
Yes. One short article gives a working vocabulary for any two species that live closely together, backed by named, checkable examples from coral reefs, deep-sea vents, legume roots and the surface of a shark.
